A study of the concentration profiles of platelets and red cells (RBCs) in tubular blood flow is proposed. Past work showed that platelet distributions may be highly non-uniform; concentrations near the wall can reach five times the core concentration. Such platelet distributions, which are not predicted by mass transfer theory, are caused by the flow-induced actions of RBCs. Thorough descriptions and models of the near-wall excesses are fundamental to understanding blood-surface interaction, thrombosis and hemostasis as their mechanisms depend upon the number and length of exposure of platelets at reactive sites. The primary experimental method, developed and demonstrated in the last two years, captures the distribution of fluorescent platelet-sized latex beads in a blood suspension flowing through a polyethylene tube by rapidly freezing it with liquid nitrogen. Segments of the frozen tube are processed in a microtome mounted on an epifluorescence microscope; images of cross- sectional cut surfaces are collected on video tape. The images are subsequently analyzed to obtain distributions of the platelets. In most of the proposed work, platelets and a portion of the red cells will be labelled with fluorescent stains; methods allowing the use of whole blood will be developed. Also, the existing experimental method will be changed so both RBCs and platelets can be observed in the same cross-section and so the orientation of RBCs can be quantified from the images. Experiments will obtain distributions in rheological situations that are chosen to facilitate modelling with mass transport theory; these situations include axial development, pulsatile flow, and transient development of the local concentration. Rheological parameters to be varied among the experiments include the hematocrit, wall shear rate, tube diameter, RBC membrane deformability, and other stress-related parameters; the hypotheses to be tested with respect to the last two parameters relate to the possible need for red cell deformation in order for large near-wall excesses to occur. To examine the effect of changes of diameter, distributions will be measured before, in, and after a constriction in the tube. Such experiments simulate events in vascular spasm and permit modelling of platelet transport in accelerating and decelerating flow fields.